SUMMARY Hutchinson-Gilford Progeria Syndrome (HGPS) is a rare genetic disease of premature aging caused by an autosomal dominant mutation in LMNA, the gene encoding laminA. The mutant protein, LmnAG608G has been named Progerin. Children with HGPS typically die in their teenage years as a consequence of CVD (atherosclerosis, myocardial infarction and/or stroke). Remarkably, CVD and death occurs in the absence of high cholesterol, but the arteries of HGPS patients are abnormally stiff, and arterial stiffness has been identified as a cholesterol-independent risk factor for CVD. Moreover, we previously showed that in vivo inhibition of arterial stiffening reduces atherosclerosis in apoE-null mice. Because Progerin is aberrantly farnesylated, therapies for HGPS have focused farnesyltransferase inhibitors (FTIs), but those studies do not directly address the cardiovascular pathology that is thought to trigger early death in HGPS. We recently obtained the LMNAG609G mouse that corresponds to the LMNAG608G mutation in human HGPS and show here that this mouse phenocopies the human disease in showing premature arterial stiffening. Immunostaining of arterial sections and an ECM expression array have identified two lead candidates for this premature arterial stiffening, and those will be studied here. We also found that HGPS arteries are deficient in their response to vasoconstrictors, and our preliminary results link this contractility defect to a striking uncoupling of two well established smooth muscle differentiation/CArG genes: expression of smooth muscle myosin heavy chain (SM-MHC) is reduced in HGPS while smooth muscle actin (SMA) levels are relatively normal. Importantly, we have been able to recapitulate this in vivo phenotype of uncoupled SM-MHC vs. SMA expression in primary smooth muscle cells (SMCs) from WT and HGPS aortas. SM-MHC is among the most important regulators of the high contractility state in differentiated SMCs. Thus, these findings, and related traction force microscopy (TFM) experiments in Preliminary Studies, lead us to a new model for premature arterial stiffening in HGPS: the expression of mutant LaminA (Progerin) leads to a preferential downregulation of SM-MHC, and this locks HGPS SMCs into a novel intermediate tensional state in which they behave more like a de-differentiated SMC, producing ECM proteins and ECM remodeling enzymes that lead to an acceleration of arterial stiffening. Aim 1 will use age-matched WT and HGPS mice to test for causal relationships between i) ECM remodeling events and premature arterial stiffening and ii) SM-MHC expression and arterial ECM remodeling. Aim 2 will use isolated SMCs to identify molecular mechanisms and causal relationships between HGPS, SM-MHC expression, cellular contractility, and ECM remodeling. It will also establish molecular mechanisms by which expression of Progerin, and possibly WT LaminA, affects SM-MHC gene expression. Aim 3 will test for similarities and differences between arterial stiffening, ECM remodeling and SM-MHC/SMA expression in normal aging vs. HGPS.